Ukrainian President Volodymyr Zelenskyy on Saturday called again for a meeting with Russian leader Vladimir Putin in an effort to "put an end to the war."

"I think that whoever started this war will be able to end it," he told a news conference at a metro station in the heart of the Ukrainian capital, adding that he was "not afraid to meet" Putin if it would lead to a peace deal between Russia and Ukraine.

"From the beginning, I have insisted on talks with the Russian president," he said. "It's not that I want (to meet him), it's that I have to meet him so as to settle this conflict by diplomatic means.

"We have confidence in our partners, but we have no confidence in Russia," he added.

Zelenskyy also repeated his warning that they would break off talks if Russia killed the remaining Ukrainian soldiers in the besieged Black Sea port of Mariupol.

"If our men are killed in Mariupol and if these pseudo-referendums are organized in the (southern) region of Kherson, then Ukraine will withdraw from any negotiation process," he said.

He was ready to exchange Ukraine's soldiers defending the city "in whatever format" to save "these people who find themselves in a horrible situation, surrounded."

He said the "last contact" with the Mariupol soldiers had been an hour ago, he said, adding "today is one of the hardest days" since the start of the Russian siege of the city at the beginning of March.